1. Field of the Invention
This invention relates to zoom lenses suited to still cameras, cine cameras, and video camers, and more particularly to long focal length zoom lenses of extended range.
2. Description of the Prior Art
Recently, in many fields of art, there have been increasing demands for zoom lenses of high zoom ratios or extended range.
To achieve a great increase in the zoom ratio, the lens designers have generally courted the method of increasing the total movement of the zoom lens units, or the method of strengthening the refractive powers of the zoom units, or the method of increasing the number of zoom units to, for example, 3 or 4. Of these, the first method is unavoidably associated with a disadvantage of increasing the total length of the lens. The use of the second method results in increasing the amount of aberrations produced, making it difficult to maintain proper stability of aberration correction throughout the extended zooming range.
The employment of the third method is disclosed in Japanese Laid-Open Patent Application No. SHO 57-2014 wherein the zoom lens has five lens units, all movable for zooming. This zoom lens may be considered derived from the prior known 2-component zoom lens whose front component is of positive power and rear component is of negative power with the separation therebetween being variable to effect zooming. That is, the front component is divided into two lens units of which the first counting from the front has a positive refractive power, and the second has a negative refractive power, and the rear component is divided into three lens units which are respectively positive, negative and positive in refractive power. And, when zooming, the separation between the first and second lens units increases with the advantage that even the second lens unit contributes to an image magnification varying effect. Furthermore, the third, fourth and fifth lens units are moved in such a way as to increase the image magnification varying effect of the entire system. Thus, a zoom lens of high range is realized. In this lens, the rays of light arriving at the third lens unit are of divergence, and are then subjected to convergence, divergence and convergence successively in passing through the third, fourth and fifth lens units respectively. Such a zoom type is, therefore, advantageous in ensuring the necessary back focal distance for wide angle zoom lenses, but in application to telephoto zoom lenses, tends to increase the length from the front vertex of the third lens unit to the image plane with an increase in the total length of the zoom lens.
The method for obtaining focal lengths beyond the variable focal length range is also known, by constructing the zoom lens in tandem form in U.S. Pat. No. 4,033,674. That is, the front zoom section is followed by a stationary lens component, a compensator, and variator and an image forming lens component which constitute the rear zoom section. Though this form has the merit of providing the possibility of operating the front and rear zoom sections simultaneously, the disadvantage is that it increases greatly the total length. For this reason, that zoom lens is adapted to be used in full size television cameras which work in broadcasting stations, and its form has never been employed in designing lenses for single lens reflex cameras and small size video cameras.
Mention should also be made of another known method by using an attachment lens in a space of the image forming lens component of the zoom lens. But, the creation of this space calls for a large increase in the total length of the lens. Another problem is that when the attachment lens is not in use, another space that accomodates it is necessitated. Therefore, this method cannot be advantageously used in interchangeable lenses.
An object of the present invention is to provide a zoom lens capable of greatly varying the focal length.
Another object is to provide a zoom lens of short total length with high performance.
Still another object is to achieve longer focal lengths than the longest of the peculiar focal length range.